A Cessna 208B Grand Caravan operated by Federal Air sustained substantial damage in a taxi accident at Mala Mala Airport (AAM), South Africa. None of the 13 occupants were injured. The captain, pilot flying, reported that after start up on the apron he taxied the aircraft towards the threshold of runway 16 and everything was normal. As the aircraft began moving down the hill on the taxiway for runway 16, the aircraft drifted off to the left of the centre line. He applied corrective action to the right but was unsuccessful. The aircraft continued drifting towards the left and departed the taxiway. He then applied maximum brakes but was unsuccessful. He then requested the first officer to apply brakes on the right hand side but this was also unsuccessful. The aircraft continued rolling, ended up in a ditch and collided with an embankment. Investigation revealed no anomalies with the rudder/brake system. PROBABLE CAUSE: “Poor airmanship.”